The invention relates to an apparatus for the combustion of vanadium-containing fuels.
Vanadium-containing fuels are obtained as residues during pretroleum refining. These residues are generally burned in spiral flow or rotary heaters and vanadium and compounds thereof, together with other recyclable constituents of the residues are obtained in slag and ash form, which can advantageously undergo further treatment. Simultaneously the heat released during combustion can be recovered.
A spiral flow heater for the heat treatment of carbon-containing residues from petroleum refining is known from DE 41 14 171 C2. Carbon-containing materials with non-flammable constituents and pollutants are supplied in a predetermined particle size tangentially in a delivery air flow to a combustion chamber and burned at temperatures above the slag melting point. The combustion air is tangentially blown in such a way that a direct contact and sticking of slag to an inner lining of the combustion chamber are avoided. As a result of cooling, the slag is discharged in solid form.
It has been found that during the combustion of vanadium-containing carbon black dust in a spiral flow furnance spontaneously liquid slag is produced at the common burner chamber temperatures.
The spontaneous slag sticking is particularly disadvantageous in the vicinity of the supply nozzles for the pulverized fuel-air mixture and the air nozzles for the combustion air. Even after relatively short operating periods the nozzles suffer slag penetration leading to a restriction and disturbance to heater operation.
The object of the invention is to provide an apparatus and a method for the combustion of vanadium-containing fuels, particularly from petroleum refining, which permit a substantially troublefree and particularly efficient recovery of vanadium and hot gas production without any slag penetration of the feed nozzles.
According to the invention, this object is achieved by an apparatus having a combustion area, a start burner and feeds for a pulverized fuel-air mixture and combustion air, as well as with a flue gas outlet and slag discharge means, in which a top burner is located above the combustion area and is formed in a top cover as a top cover burner. In the cover of the top burner are located at least the start burner and the supply for the pulverized fuel-air mixture with at least one dust nozzle. The at least one dust nozzle is positioned in such a way that the pulverized fuel-air mixture is introduced into the combustion area on a secant to the cross-sectional surface thereof and under an angle between 35xc2x0 and 65xc2x0 to the longitudinal axis of the combustion area or alternatively coaxially to the start burner.
From the method standpoint, the object is achieved in that the vanadium-containing residues from petroleum refining or also other vanadium-containing fuels are fed to a top burner, which is located in a top cover of a combustion area. According to the invention, the pulverized fuel-air mixture is supplied following a secant to the cross-sectional surface of the combustion area and under an angle between 35xc2x0 and 65xc2x0 to the longitudinal axis thereof or, in an alternative construction, coaxially to a start burner located in the top cover of the combustion area and is burned with short burn-out times and an adjustable ignition front.
The method and apparatus according to the invention are based on the surprisingly high reactivity of the vanadium-containing residues and an extremely rapid ignition and short burn-out times of the vanadium-containing pulverized fuel. Tests have shown that the high reacitivity and high combusiton speed and the formation of a highly corrosive, liquid slag can be attributed to metallic constituents of the fuel, which oxidize. It is assumed that the metallic constituents have a catalytic action on the combustion and bring about the formation of the spontaneous, liquid slag. During combustion vanadium is converted into vanadium pentoxide, which has a melting point of 672xc2x0 C. In mixtures with further metal oxides, e.g. nickel and iron oxides, there is a slag melting point between 700 and 850xc2x0 C., which is extremely low compared with other slags. Therefore, the possible combustion temperatures are always above this melting point, so that basically, liquid slag is unavoidable. In order to ensure that no function-preventing sticking occurs in a combustion chamber and particularly in the vicinity of the feed nozzles for the pulverized fuelair mixture, for combustion air or other media, according to the invention a top cover or roof burner is provided and the feed nozzles are oriented in such a way that a return flow of liquid slag is prevented. With a defined flow guidance and nozzle shaping a substoichiometric combustion zone is obtained directly after the pulverized fuel has passed out of the dust nozzles, so that spontaneous slag formation in the vicinity of the dust nozzles is prevented. Moreover, the defined flow guidance makes it possible to endure an adequate, predeterminable spacing between the nozzles and the formation of liquid slags.
In a first apparatus embodiment the dust burner is constituted by a top burner in a top cover with a frustum-like cover wall. Eccentrically within the top cover are provided a predeterminable number of dust nozzles, which are constructed in lance-like manner. By means of said dust nozzles the pulverized fuel is blown in on secants under a predeterminable angle to the longitudinal axis of the refractory lined combustion chamber. Directly after passing out of the dust nozzles no secondary combustion air is supplied, so that there are near-stoichiometric to pronounced substoichiometric ratios in said first combustion zone, e.g. with xcex=0.2 to 1.0.
The arrangement of the dust nozzles or lances in the cover of the combustion chamber prevents a clogging of the nozzles with slag in this embodiment. It is advantageous that the dust exit velocity can be modified for changing the ignition front of the dust in a predeterminable spacing with respect to the nozzle. Appropriately the velocities of the pulverized fuel supplied are between 10 and 45 m/sec, preferably 20 m/sec.
Tests have shown that the reactivity of the vanadium-containing fuel is decisively dependent on the vanadium and oxygen content of the residues. With a lower vanadium and oxygen content it is advantageous to compensate the slower reaction speed by a better mixing of the pulverized fuel and the air. In respect of the apparatus, such a mixing can be implemented by spin means or swirling devices in the preferably annular combustion air duct of the top or roof burner.
For the supply secondary air to the combustion chamber there can be provided a stepped air supply by means of several, preferably two air nozzles. Specially shaped flaps in the air nozzles make it possible to change the exit velocity of the combustion air for different mass flows. Thus, the ash/slag ratio can be varied. In addition, the combustion air exit velocity effects the burn-out, so that the latter can also be controlled via the exit velocity of the combustion air.
In a second apparatus embodiment a top burner is placed in a roof or cover of a combustion area which, as in the first apparatus and method embodiment, is formed in a refractory lined combustion chamber. Thus, the top burner is located in a cover of the refractory lined combustion chamber and can also be referred to as a cover or head burner.
It is advantageous that the refractory lining of the preferably cylindrical combustion chamber can serve as an ignition aid and a double jacket for preheating the combustion air. Combuation is largely ended within a relatively small combustion chamber volume. A waste heat boiler, which follows the refractory lined combustion chamber, can have a smaller volume than when no lined combustion chamber is used. Obviously cost advantages result from this solution.
It is also appropriate for the top burner to have a start burner, which is preferably operated with gas or oil and as a dust nozzle is provided an annular clearance for the vanadium-containing pulverized fuel-air mixture in concentric manner around the start burner.
Combustion with a top burner in a refractory lined combustion chamber is carried out at temperatures in the range 1100 to 1650xc2x0 C., preferably at 1200xc2x0 C. It has been found that the vanadium-containing residues ignite at a safe distance upstream of the top burner and, aided by the refractory walls of the brick lined combustion chamber, a volume of maximum combustion intensity is formed. This leads to an almost complete and rapid burning of the fuel, which in respect of the apparatus is advantageous for the after-reaction volume of a first flue pass of a downstream waste heat boiler. The flue gas formed during combustion, together with the liquid slag pass into the waste heat boiler, where the slag is cooled to temperatures below the solidification point of approximately 800 to 900xc2x0 C. and are advantageously largely discharged as very fine dust together with the flue gas.
For the case a solid slag is obtained, it is appropriate to provide the waste heat boiler and in particular the first flue pass with a slag discharge means, so that slag droplets deposited on the refractory walls of the combustion chamber and which drop into the waste heat boiler can be discharged.
In a third apparatus embodiment a combustion area with upstream top burner is placed in a waste heat boiler. The top burner is incorporated into the roof of the first flue pass of the waste heat boiler. There is no need to have an ignition aid in this apparatus and method embodiment. Thus, in respect of the method, the top burner is operated for obtaining higher combustion temperatures with a lower air excess. The air excess is in the range xcex=1.05 to 1.4, preferably at xcex=1.1. If combustion takes place at temperatures of 1600 to 1800xc2x0 C., a good burn-out can be obtained.
If no stable flame forms, it is advantageous to use the centrally positioned start burner with a lower load as a supporting burner.
As a result of the high combustion temperatures, liquid slag is also produced in this apparatus embodiment. The slag droplets are finely dispersed in the flue gas and cool in the flue flow through a radiant heat exchange with the boundary walls of the waste heat boiler. Thus, the vanadium-containing fuel is discharged almost completely as pulverulent slag. There is consequently no need for a removal of solidified slag, which is generally complicated.
Appropriately recirculated flue gas is vertically injected by means of nozzles into the boiler top cover and concentrically to the top burner or head burner. Slag droplets on the walls of the waste heat boiler are repelled by the injected, recirculated flue gas and caking on the waste heat boiler walls is prevented. Prior to entering pipe bundles, which are in particular located in a third flue pass of the waste heat boiler, the flue gas and the slag constituents contained therein are cooled to below 500xc2x0 C., to prevent corrosion, particularly due to vanadium oxides, especially vanadium pentoxide.
According to an advantageous development use is made of a combustion chamber, which at least in the particularly wear-intensive areas, has coolable walls or wall sections as a so-called xe2x80x9ccooling fieldxe2x80x9d. The cooling field can be formed by water-containing pipes in a refractory lining of the combustion chamber. For example, pinned pipe coils can be laid horizontally and enveloped with a refractory vibration material. The intense, water-side cooling ensures a cooling of the combustion chamber-side surface to a temperature below the solidification point of the downwardly flowing slag and the formation of a corrosion-protecting slag shield.
It is appropriate to only construct the particularly wear-intensive wall areas as xe2x80x9ccooling fieldsxe2x80x9d in order not to disadvantageously influence the heat balance of the combustion chamber through heat losses via the walls. Preferably, no more than 15% of the entire surface of the combustion chamber should be constructed as a xe2x80x9ccooling fieldxe2x80x9d.
According to a further apparatus and method embodiments, the distance of the ignition front from the pulverized fuel-air mixture supply is regulated by an enveloping of the dust jet. The enveloping can be constituted by an inert gas, e.g. nitrogen. As a result of the inertizing envelope around the dust jet it is possible to prevent premature ignition in the vicinity of the dust nozzle and this can be regulated by admixing the combustion or secondary air.
It is also advantageous that slag caking is further prevented through the exit velocity of the enveloping inert gas.